The advent of digital lighting technologies, i.e. illumination based on semiconductor light sources, such as light-emitting diodes (LEDs), offers a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, robustness, lower operating costs, and many others. For example, LEDs are particularly suitable for applications requiring small or low-profile light fixtures. The LEDs' smaller size, long operating life, low energy consumption, and durability make them a great choice when space is at a premium.
A “downlight” is a light fixture that is installed into a hollow opening in a ceiling and often referred to as a “recessed light” or “can light.” When installed, it appears to concentrate light in a downward direction from the ceiling as a broad floodlight or narrow spotlight. Generally, there are two parts to recessed lights, the trim and housing. The trim is the visible portion of the light and includes the decorative lining around the edge of the light. The housing is the fixture itself that is installed inside the ceiling and contains the light socket.
An alternative to recessed lights is a surface-mount or suspended downlight, combining the functionality of the latter with flexibility and ease of installation over conventional junction boxes, particularly where disposal of the recessed light housing in the ceiling is impractical. In that regard, architects, engineers and lighting designers are often under considerable pressure to use low-profile, shallow-depth fixtures. Fundamentally, floor-to-floor heights are limited by developers looking to maximize their floor-to-area ratio; yet designers want to maximize the volume of the space by including the tallest ceilings possible. This contradiction sets up a conflict between various utilities, including lighting, that are competing for the limited recess depth found between the finished ceiling and the structural slab above.
Designers have also shunned most surface-mounted general-illumination solutions; the size of the primary light sources and ballasts, along with required optics and glare shielding techniques, quickly makes the fixtures too large to be aesthetically acceptable to most designers. Also, the compromises made to achieve low profile mounting heights in fixtures with traditional light sources typically negatively impact overall fixture efficacy. In fact, total fixture efficacy for many surface mounted compact fluorescent units averages only 30 lm/w.
A further deficiency with conventional downlights is that their large size can preclude their use for emergency lighting. That is, the addition of a backup power supply within the conventional fixture would make the fixture too large to be aesthetically acceptable or to fit within the allotted ceiling space. In conventional lighting schemes, only a selected few, if any, of the general illumination lights in an illuminated space may be provided with back-up power. Alternatively, a completely separate lighting system must be implemented for emergency lighting needs, thereby adding costs and space requirements.
Thus, it is desirable to provide a downlight fixture employing LED-based light sources that addresses a number of disadvantages of known LED illumination devices, particularly those associated with thermal management, light output, and ease of installation. Accordingly, one object of the invention disclosed herein is to provide a shallow surface-mount fixture—as shallow as 1″-2″ overall height—to alleviate the undesirable constraints of shallow recess depths for many designers; in fact, it could help many projects reclaim up to 6″ of ceiling height. Additionally, it would offer an elegant solution to projects with no recess cavity at all (mounting directly to concrete slabs). Another object is to achieve an overall fixture efficacy of about 30 lm/w or better in order to set various implementations of this invention on an equal plane with fluorescent sources yet at output levels normally associated with incandescent fixtures, thus setting this fixture up well for environments with low ambient light levels.
Additionally, maintaining a proper junction temperature is an important component to developing an efficient lighting system, as the LEDs perform with a higher efficacy when run at cooler temperatures. The use of active cooling via fans and other mechanical air moving systems, however, is typically discouraged in the general lighting industry primarily due to its inherent noise, cost and high maintenance needs. Thus, it is desirable to achieve air flow rates comparable to that of an actively cooled system without the noise, cost or moving parts, while minimizing the space requirements of the cooling system.